1. Field of the Invention
The present invention relates to a system for injecting a fuel into internal combustion engine and, more particularly, to a fuel injection system for an internal combustion engine adapted to control the rate of the fuel injection in linear relation with the varying flow rate of intake air induced into the engine.
Description of the Prior Art
A fuel injection system of the kind specified is disclosed, for example, in U.S. Pat. No. 3,996,910 and, therefore, has been publicly known. This known fuel injection system includes an air metering plate disposed in the intake duct of an engine and adapted to be angularly displaced in response to the change of intake air flow rate. The angular displacement of the air metering plate is mechanically transmitted through a link mechanism to a plunger so as to axially displace the latter thereby to effect a distribution and metering of the fuel.
This known fuel injection system, however, has a drawback that the size of the whole system is large due to the fact that an intake air sensor having the air metering plate and a fuel controlling mechanism for performing the distribution and metering of the fuel are assembled together into a unitary structure. Thus, the known fuel injection system inconveniently occupies an impractically large space in an engine compartment of an automobile.
The known fuel injection system, moreover, requires a highly complicated link mechanism for obtaining a linear relation between the angular displacement of the air metering plate and the axial reciprocative movement of the plunger. In addition, the desired linear relation can hardly be obtained, evey by such a complicated link mechanism, because of the mechanical play involved in the latter.
As a measure for overcoming above-stated problems, an improved fuel injection system has been disclosed in U.S. patent application Ser. No. 693,951 of the same assignee (Nippon Soken, Inc.) as that of the present application, now issued as U.S. Pat. No. 4,040,405.
In this improved fuel injection system, the angular displacement of an air metering plate caused by a change of the intake air flow rate is converted into a hydraulic pressure signal. A control shaft adapted to determine the fuel injection rate is axially moved in accordance with the hydraulic pressure signal to control the rate of the fuel injection.
This newly proposed system overcomes the above stated problems to a certain extent. However, this fuel injection system has another disadvantage. More specifically, the hydraulic pressure signal according to the metered intake air flow rate is applied to one axial end of the control shaft, while the other end of the shaft is subjected to a return spring force which acts in the counter direction to the hydraulic pressure signal, so that the control shaft may be axially moved to a position where the axial forces caused by the hydraulic pressure signal and the spring force balance. The use of the spring for determining the axial position of the control shaft is apt to incur a deterioration of the accuracy or precision of the fuel control because there may be a fluctuation of spring constant of the springs and because the spring constant of a spring is varied due to the secular variation. Further, from a practical point of view, it is extremely difficult to obtain a hydraulic pressure signal which acts on one axial end of the control shaft substantially in proportion to the intake air flow rate. Thus, it is materially impossible to obtain the axial displacement of the control shaft in exact proportion to the intake air flow rate because the spring return force is in correct proportion to the axial displacement of the control shaft, while the counter hydraulic pressure signal, as stated above, cannot correctly be proportional to the intake air flow rate.